kernel.bbclass: Update cleandirs to remove new kernel staging dirs
Fixes [YOCTO 6818]
Update do_unpack[cleandirs] to include the STAGING_KERNEL_BUILDDIR, and
add the same set of updated cleandirs for do_clean.
Signed-off-by: Darren Hart <dvhart@linux.intel.com> Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com> Signed-off-by: Ross Burton <ross.burton@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
